1. Field of the Invention
2. Description of the Related Art
This invention relates to combustors for gas turbine engines and particularly to multi-burner combustors. Such combustors for high efficiency turbines operate in onerous conditions, having to withstand gas exit temperatures in the region of 1200.degree. C. It is therefore necessary to provide cooling means for the combustor which, while being effective, does not detract excessively from the performance of the combustion system.
It is also necessary to take account of different turbine loading conditions, between idling and full load, which make different demands on the combustor. The proportion of fuel to combustion air has to be varied greatly throughout the loading conditions and at very low loads a problem arises in trying to maintain stable combustion with very low fuel supply. The use of multiple burners in a single combustion chamber does offer a partial solution to this problem in that the fuel feed to some burners can be maintained at a reasonable level while other burners can be turned off completely.
This does, however, require more complex contol of the fuel feed system and is to be avoided if possible. This `solution` does also produce greater non-uniformity of temperature distribution and consequent thermal stresses resulting in shorter operational life. If this `staging` of the fuel supply is not employed, there are still advantages in a multi-burner arrangement, which could derive from its combination with other features:
(a) a multiplicity of fuel injection points spread across the upstream entrance to a flame tube, in combination with a uniform cooling of the flame tube walls, provides the possibility of good control of temperature distribution, and particularly turbine entry temperature distribution, resulting in longer operational life; PA0 (b) a multi-burner arrangement combined with an unconventionally large proportion of compressed air admitted for primary combustion provides conditions leading to a significant reduction in emission of oxides of nitrogen; PA0 (c) a multiplicity of fuel injection points uniformly distributed across the flame tube entrance provides an improved fuel/oxidant mixing geometry for inerts-laden, low heating value fuel.
Some attempt at control of the low fuel operating condition and maintenance of stable combustion has been made by feeding combustion air in jets through fairly large holes in the combustor wall and using gaps between successive concentric wall sections, as illustrated in FIG. 1 of the accompanying drawings, for the introduction of film cooling air. While this induced turbulence, in conjunction with the common use of a single fuel injector and primary air swirler, does facilitate the flame stabilisation function, there is a resultant loss of effective and uniform cooling of the combustor walls. The injection of penetrating air jets from the wall of the flame tube tends to cause disruption of any cooling film at the wall surface and consequent temperature variations. Thermal distortion and/or erosion results and the flame tube suffers a reduced life.